1. Field of Invention
The present invention relates to susceptors and to structures for supporting susceptors in chemical vapor deposition (CVD) reactors, and more particularly, to such susceptors and structures for use in single-wafer RF-heated reactors.
2. Description of Related Art
In conventional silicon wafer processing, multiple silicon wafers are placed onto a susceptor within a chemical vapor deposition (CVD) reaction chamber for heating and processing. However, because of increased wafer sizes, i.e., up to 12 inches in diameter, a recent trend has been to process these wafers individually using single-wafer CVD reactors. Such reactors also greatly reduce the time necessary to heat the wafer to the processing temperature.
In processing a wafer, it is desirable to deposit uniform film layers onto the wafer, which requires the wafer be uniformly heated and maintained at a uniform temperature. One method of heating the wafer is through the use of heat sources, such as high intensity heat lamps, positioned external to the reaction chamber. Large amounts of energy from these lamps radiantly heat the wafer and a susceptor on which the wafer lies. The lamps can be placed at locations to provide uniform heating and constant temperature gradients across the wafer. However, in these types of systems, the lamps also heat the walls of the reactor chamber, which causes process gases to deposit on the walls as well as on the silicon wafer. The resultant film formed on the chamber walls absorbs some of the radiant energy emitted from the heating lamps and thereby locally increases the temperature of the chamber walls. As a result, process gases deposit on the chamber walls at an increasing rate. Therefore, the chamber walls must be carefully etched or cleaned, sometimes after every run. Yet, even if cleaned after a single run, film deposition on the chamber walls may still affect the temperature uniformity within the run.
Another method for heating a wafer in single-wafer CVD reactors, which overcomes the problems associated with radiant energy techniques, uses radio frequency (RF) as the heat source. In such systems, RF induction coils may be positioned, for example, underneath a susceptor holding the wafer. The susceptor, typically made of silicon carbide-coated graphite, absorbs energy from the RF coils, thereby heating the susceptor. This heat energy is then transferred by the susceptor to the wafer to bring the wafer to the desired processing temperature, with the amount of heat transfer dependent on both the emissivity and conductivity of the susceptor. Therefore, in contrast to prior methods, the heat transferred to the wafer is independent of the thickness of deposited layers formed on the chamber walls. Commonly-owned U.S. Pat. No. 5,653,721, entitled "GAS INJECTION SYSTEM FOR CVD REACTORS", describes such a single-wafer RF heated CVD reactor and is incorporated herein by reference in its entirety.
The above-described RF heated CVD system decreases the effect of silicon layer formation on chamber walls. However, with systems using RF energy, the RF energy, from the coils for example, exerts a lifting force on the susceptor. This force can be greater than 40 pounds, which may result in the lifting and disengaging of the susceptor from its moorings during the heating process.
Furthermore, a system where a wafer is cooled through heat transfer from the wafer to a susceptor to a heat sink, such as the one incorporated by reference above, can be improved by increasing wafer cooling efficiency. For example, a system may include RF coils housed in a quartz structure with a silicon carbide plate on top of the housing and below the susceptor. After the heating process, water running through the RF coils cools the coils, which in turn cools the silicon carbide plate. This plate acts as a heat sink for the susceptor to facilitate the cooling phase in the CVD process. To effectively utilize the heat sink, the susceptor must be brought in contact with the silicon carbide plate. Without contact, heat transfer is substantially reduced. However, if the plate and susceptor are forced together after initial contact, i.e., overdriven, either or both may break due to the brittle nature of silicon carbide. As a result, a very precise and controlled movement is required for cooling the wafer without damaging reactor components.
Yet another shortcoming of a single-wafer RF heated system is that of temperature uniformity across the center of the wafer. Because no magnetic field lines exist at the center of the RF coils, the center of the susceptor is not heated, which results in the center of the wafer not being adequately heated. This problem does not arise with other types of systems. For example, with batch reactors processing multiple wafers, the middle of the susceptor is not crucial since the wafers lie outside the susceptor center, and with single-wafer reactors using radiant energy, the wafer heating is direct so that no dead zone exists in the center of either the wafer or susceptor.
Accordingly, a structure is desired which overcomes the problems discussed above with respect to a single-wafer reactor using RF energy heating.